Resource recovery: the burning issue

Scientists, engineers and economists have long pondered the issues of using waste to produce energy, but mainly in a municipal context. In the first of our seven-part series, Robin Holder, Tal Golesworthy and Peter Davies of consultants Environmental Development Technology, explore the potential of Waste to Energy for industry, looking at where we are, where we have come from and where, if the conditions are right, we might be going.

Until the mid-60s, most incineration plants were of the multiple-hearth variety with minimal gas cleaning plants. However, by the late 60s, new technologies were being introduced which included moving grates to burn the waste, with gas cleaning equipment, such as cyclones or electrostatic precipitators, to remove particulates.


Due to the perceived need to reduce the volume of municipal solid waste (MSW), about 50 new incineration plants were built during the 70s but less than 20% of these incorporated heat recovery. While most of these plants were operated with a reasonable amount of success, those with heat recovery gave the most problems – the waste heat boilers suffering from corrosion and fouling of the gas passes.

By the early 80s, local councils had less to spend, and this, combined with the abundant availability of landfill sites, produced a trend away from incineration back to landfill. The economics showed that incineration was costing about four times that of landfill and this situation continued for about 10 years until a number of factors started to change the attractiveness of the landfill option.

Public concern

There was much public concern over the leaching of effluent from landfill sites and the operators of these sites were very concerned that they would be tied in to long term 'duty of care' agreements. Also, the introduction of the Non-Fossil Fuel Obligation (NFFO) programme enabled companies producing electricity from non fossil fuel sources to secure favourable deals with the regional electricity companies (RECs). These factors, combined with the advent of much improved incinerator and gas cleaning technologies from Europe, made incineration with heat recovery and power generation a more viable option.

Comparison of the economics showed that the difference in cost between landfill and incineration with power generation/heat recovery was much narrower – typically £20/tonne for incineration and £10/tonne for landfill (depending on waste material, transportation costs, etc.). The gap had closed sufficiently for the waste companies to re-investigate incineration as an option.

By the late 80s, the majority of incineration plants were coming to the end of their useful lives and were in serious need of modification to meet the new UK emissions legislation – Environmental Protection Act, 1990. The Act divided processes into two parts: the large, more hazardous processes controlled by the HMIP (now the Environment Agency) under Part A; the smaller, 'less polluting' processes under local authority control in Part B. In order to incinerate any waste stream, the process needed to be authorised and an important part of the authorisation was that the process must use the best available technology not entailing excessive cost (BATNEEC) to minimise the emission of pollutants.

Depending on the type of waste stream being burned, typical emissions are:

particulates – dust, ash etc

acid gases – oxides of nitrogen (NOx), oxides of Sulphur (SOx), Hydrogen chloride (HCl)

unburned hydrocarbons – dioxins, PAH's, PCCB's, VOC's

heavy metals – Pb, Ni, Cd, Hg

Gas cleaning

At that time, there was little appropriate gas cleaning expertise in the UK and it was necessary to seek solutions from Europe and Scandinavia where gas cleaning technologies were already established. The nature of these was diverse. Some used wet scrubbing and others dry scrubbing to remove particulates and gaseous pollutants. Often the cleaning was multi-stage: scrubbing to remove the acid gas pollutants followed by filtration to remove particulates followed again by activated carbon filters to remove organic material and heavy metals. In most cases, the gas cleaning plant was large and expensive providing the would-be purchaser with a real dilemma when it came to evaluating the technical and commercial risk. Initially wet scrubbing was the favoured option but now the majority of recently installed UK incineration plants use dry scrubbing with fabric or ceramic filtration to give the desired emission control performance.

Combustion equipment was already well established in the UK. The larger MSW incinerators employ a moving hearth or grate while the smaller incinerators, for disposing of hazardous or clinical waste, generally comprise rotary furnaces with secondary combustion chambers. Recent additions to the market include fluidised bed combustion (FBC) and in particular circulating fluidised bed combustion (CFBC). The advantage of this technique is that the waste is well mixed and combustion residence times are long so that combustion is virtually complete. There are now several of these systems operating in the UK and overseas burning a range of waste streams normally in conjunction with a primary fuel source such as coal – an example of co-firing.

Economics

The economics of waste incineration are improved by heat recovery for process steam, power generation for site use or export to the grid, or a combination of heat and power production (CHP schemes). When these options are combined with a NFFO subsidy and a gate fee for the waste stream, the economics can be very attractive. There are, however, a number of problems in recovering heat from incineration processes. The waste is generally variable which means temperatures are difficult to control. The dust burden in the flue gas is high and often includes many chemical species which contribute towards fouling and erosion of the plant internals. Research has shown that metallic surfaces within the incinerator plant can act as a catalyst in the formation of dioxins. Unstable combustion can lead to a localised reducing atmosphere in the boiler which will cause corrosion. It is important therefore to use a system design which minimises these problems.

What, then, is the current position? Many of the older incineration plants (both clinical and MSW) have been shutdown because they were unable to meet the new emissions legislation. A few large MSW incineration plants with heat recovery and power generation (to take advantage of NFFO subsidies) have been built and are operating successfully. Some new clinical waste incineration plants are now in operation and, with mountains of rendered animal waste to get rid of, new incineration plants to burn this waste are starting to be introduced. However, gaining authorisation and planning consent to build such plants has become a protracted procedure and hindered the progress of many new schemes. The NIMBY attitude is prevalent and with pressures to prevent building on green field sites, gaining planning consent will only become more difficult.

Changing the balance

The recently introduced landfill tax will further tip the economic balance towards incineration/heat recovery/power generation but there are still barriers. Administrative problems, such as obtaining authorisation and planning, technical problems, such as complying with the ever tighter emission limits and finding long term, high calorific value waste streams make it unlikely to be as beneficial as it might seem. Landfill gas is being used in CHP schemes and looks to be an attractive option. This maybe a misconception however, as much of the gas has a low heat value and most landfill sites are away from urban areas where the heat (or power) can usefully be used.

One solution is to co-fire selected waste streams with conventional fuels into existing industrial power, heating or drying plants. Technically, most boiler/furnace plant operate at high combustion temperatures >1,000C° with long furnace residence times. This makes them suitable for burning waste materials. Many have materials handling and gas cleaning equipment already installed, or are in a position to retrofit such equipment. In fact, many companies used to burn their waste streams in their conventional coal-fired boiler plant. This was successful particularly if the waste stream was of a high calorific value because it not only got rid of the waste but it recovered useful heat and required less primary fuel.

This practice continued until the introduction of tighter emissions legislation and the need to gain authorisation. Most companies did not have adequate gas cleaning equipment to satisfy the new legislation and, having looked at the economics and technical risk, decided to landfill rather than upgrade their existing equipment.

It is clear that companies will not invest unless the economics are favourable. But there are encouraging signs. The Government had made it clear that it supports CHP schemes and is now considering its plans to increase the electricity generated from renewable resources. Now is the time to urge them to address the co-firing issue, so that companies are encouraged to burn their high calorific value waste streams on-site as efficiently and cleanly as possible. With the right technological choices, it can be done.


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